1. Field of the Invention
The present invention relates to a slurry for multi-material chemical mechanical polishing (CMP) and, more particularly, to a slurry which, in one aspect, is capable of CMP alumina and nickel iron (NiFe) to a common plane and, in a second aspect, is capable of CMP alumina, nickel iron (NiFe) and copper (Cu) to a common plane.
2. Description of the Related Art
Chemical mechanical polishing (CMP) is a process wherein a slurry is dispersed on a CMP tool for chemically and mechanically polishing different levels of different materials on a workpiece to a common plane. The CMP tool has a turntable with a pad on it and a holder which is capable of pressing a workpiece against the rotating turntable while the slurry is dispersed on the turntable. The slurry contains an abrasive material as well as a reactive material. The abrasive material may reduce the level of one of the materials of the workpiece while the combination of the abrasive and the reactive material reduces the level of another material of the workpiece. The hoped for situation is that the slurry will enable the CMP to remove the two materials on the workpiece at substantially the same rate so that the levels of these materials can be reduced to a common plane.
Chemical mechanical polishing will be especially useful for planarizing rows and columns of partially completed magnetic head assemblies wherein each magnetic head assembly includes a read head portion and a write head portion. Planarization at various levels of the magnetic head assembly during fabrication promotes improved magnetics of the magnetic layers in the assembly and reduces the risk of electrical shorting between electrical leads and electrically conductive layers within the magnetic head assemblies. It also reduces surface topography by allowing subsequent photolithographic steps to attain significantly higher optical resolution.
One of the first layers formed in a magnetic head assembly is a ferromagnetic first shield layer for the read head. After constructing a nonmagnetic electrically conductive first read gap layer on the first shield layer a read sensor assembly is formed on the second read gap layer along with first and second electrically conductive leads which are connected to side edges of the sensor and extend rearwardly into the head for connection to first and second terminals outside the head. After forming a nonmagnetic electrically insulative second read gap layer on the read sensor and the first and second leads a ferromagnetic second shield layer is formed on the second read gap layer. In a merged magnetic head assembly the second shield layer also serves as a first pole piece layer for the write head. In a piggyback head, after forming a nonmagnetic electrically insulative layer on the second shield layer, a ferromagnetic first pole piece layer is formed on the insulation layer. After forming a write gap layer and a first insulation layer of an insulation stack a write coil layer is formed on the first insulation layer. The first insulation layer extends across the first pole piece layer and therebeyond to insulate the write coil layer from the first pole piece layer and the underlying first shield layer of the read head. After forming second and third insulation layers of the insulation stack on the write coil layer a second pole piece layer is formed on the write gap layer, on the insulation stack and is connected to the first pole piece layer at a backgap. The last layer is an overcoat layer on the second pole piece layer. Multiple magnetic head assemblies are typically constructed in rows and columns on a wafer substrate.
After forming the first pole piece layer it is highly desirable that the magnetic head assembly be planarized before the construction of the write coil layer. Typically, the first pole piece layer is minimized in its width in order to reduce the overall inductance of the head and thereby preserve a higher data rate during the write function. In doing so, the width of the coil layer is significantly wider than the width of the first pole piece layer. The first insulation layer of the insulation stack covers the first pole piece layer and drops over the first and second side edges of the pole piece layer and then extends laterally in both directions from the first pole piece layer. Unfortunately, where the first insulation layer makes a step at the first and second side edges of the first pole piece layer the thickness of the first insulation layer undergoes a reduction which may result in pinholes exposing portions of the first and second side edges of the first pole piece layer. Since the first pole piece layer is electrically conductive this presents a problem when the write coil layer is formed on the first pole piece layer and likewise makes first and second steps at the first and second side edges of the first pole piece layer. Further, the steps of the write coil layer at the first and second side edges of the first pole piece layer result in poorly defined vertical edges of the write coil layer. The write coil layer is frame plated which results in the photoresist frame assuming a poorly defined shape as it transcends the first and second steps caused by the first and second side edges of the first pole piece layer. Accordingly, it would be highly desirable to planarize the magnetic head assembly before constructing the first insulation layer and the write coil layer.
We have provided a slurry for polishing various levels of the magnetic head assembly to a common plane, especially the level of the first pole piece layer of a magnetic head assembly before construction of a write coil layer. After the construction of the first pole piece layer we have investigated a process wherein alumina (Al2O3) is sputter deposited over the entire wafer at a level above the top level of the first pole piece layer. The alumina has a high profile on top of the first pole piece layer and a lower profile beyond the first and second side edges of the first pole piece layer wherein the lower level is above the top surface of the first pole piece layer. The first pole piece layer is typically nickel iron (Ni80Fe20). In a first aspect of the invention we have provided a slurry which, when used with a chemical mechanical polishing tool, the rate of reduction of the levels of the alumina and the nickel iron are substantially the same so that they can be chemically mechanically polished to a common plane. In the first aspect of the invention a first concentration of colloidal silica and a second concentration of potassium persulfate (K2S2O8) or sodium persulfate (Na2S2O8) is employed with water in a slurry wherein the first and second concentrations are selected so that the alumina and the nickel iron are polished at the same rate. In both aspects of the invention the slurries enable the alumina and the one or two metallic materials to be polished to a common plane.
In a second aspect of the invention we have provided a slurry that not only polishes the alumina and the nickel iron to a common plane but also polishes copper (Cu) at substantially the same rate so that during the chemical mechanical polishing process the alumina, the nickel iron and the copper are all polished to a common plane. Before or after constructing the coil layer, first and second vias are made to the first and second leads of the read sensor and copper is formed therein to make first and second copper studs that extend toward first and second terminal sites for the read head. Accordingly, when the magnetic head assembly is polished to a common level before constructing the write coil layer it is required that the slurry have the capability of polishing the alumina, the nickel iron of the first pole piece layer and the copper of the copper studs at the same rate to a common plane. The slurry contains a third concentration of ammonium persulfate ((NH4)2S2O8) wherein the first, second and third concentrations are selected so that the alumina, the nickel iron and the copper are polished at the same rate.
A third aspect of the present invention is to provide a ratio of the third concentration to the second concentration that polishes the first and second metals at the same rate and, while maintaining this ratio, adjusting the first concentration of the silica in order to optimize the chemical mechanical polishing for increasing manufacturing yield. The invention also includes making first and second aspects of the slurry as well as employing the first and second aspects of the slurry for chemical mechanical polishing various thin film devices such as magnetic head assemblies. The materials polished are exemplary and may optionally include other materials.
An object of the present invention is to provide a slurry which can be employed for chemically mechanically polishing (CMP) alumina and a first metal, such as nickel iron, at the same rate to a common plane.
Another object is to provide a slurry for chemically mechanically polishing (CMP) alumina and first and second metals, such as nickel iron and copper, at the same rate to a common plane.
A further object is to provide a method of making a slurry which contains a first concentration of silica and second and third concentrations of reactive materials wherein a ratio of the third concentration to the second concentration polishes first and second metals of a workpiece at the same rate and this rate is maintained constant while the first concentration is adjusted to optimize the rate of chemically mechanically polishing.
Another object is to provide a method of chemically mechanically polishing an alumina overcoat layer, a nickel iron first pole piece layer and copper studs with a slurry that chemically mechanically polishes the alumina, the nickel iron and the copper at the same rate so that the first pole piece layer, the alumina overcoat layer and the copper studs are chemically mechanically polished to a common plane.
The nature, objects, and advantages of the invention will become more apparent to those skilled in the art after considering the following detailed description in connection with the accompanying drawings.